While thyroid is a dormant organ, when required, it can regenerate and undergo folliculogenesis. How this occurs remains unknown. We took advantage of using Nkx2-1(fl/fl);TPO-cre mouse that we previously produced in our laboratory to address these questions. In their thyroids, the Nkx2-1 gene is disrupted in thyroid follicular cells-specific fashion in a hypomorphic manner; their thyroid follicles consist of follicular cells that have the Nkx2-1 gene almost completely disrupted, while other follicles have no alterations in the Nkx2-1 gene. As a whole, these thyroids appear to have approximately a half of the Nkx2-1 genes deleted. NKX2-1 is one of the thyroid differentiation markers and the critical transcription factor, together with PAX8, both of which regulate development, homeostasis, and function of thyroid. The thyroids of Nkx2-1(fl/fl);TPO-cre mice are very disorganized and the follicular cells that have lost NKX2-1 expression appear to undergo degeneration. As a result of this, their thyroids have approximately a 2-fold higher proliferation rate, which contributes to a higher incidence of thyroid tumors induced by genotoxic carcinogen such as N-bis(2-hydroxypropyl)-nitrosamine (DHPN). Thyroids of these mice are thus under constant degeneration and regeneration. In the thyroids of Nkx2-1(fl/fl);TPO-cre mice, a zone of possible precursor cells to thyroid follicular cells were detected aligning near the tracheal cartilage and muscle that are thinner and elongated in shape as compared with round shape follicular cells. These elongated cells were positive for expression of NKX2-1, suggesting that they are in thyroid lineage. A newly formed follicle-like structure was found connected at the end or in the middle of the zone of elongated cells. When Nkx2-1(fl/fl);TPO-cre mice received bone marrow cells obtained from GFP-transgenic mice in bone marrow transplant experiments, some GFP-positive cells were found surrounding NKX-2-1-positive cells in the zone of cells. These results suggest that a zone of clustered cells could be, at least partly, derived from bone marrow cells. Based on these results, we propose that a cluster of the elongated NKX2-1-positive cells may be precursor cells that mature to become thyroid follicular cells, forming thyroid follicles. We also found that phosphorylation of AKT is induced by NKX2-1 in the proposed thyroid progenitor-like SPTL (side-population cell-derived thyroid cell line) cells that we previously established, suggesting the possibility that NKX2-1 plays a role in differentiation through AKT signaling. This study revealed that Nkx2-1(fl/fl);TPO-cre mice provide a suitable model to study in vivo regeneration and folliculogenesis.